'\" et
.TH MSGGET "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
msgget
\(em get the XSI message queue identifier
.SH SYNOPSIS
.LP
.nf
#include <sys/msg.h>
.P
int msgget(key_t \fIkey\fP, int \fImsgflg\fP);
.fi
.SH DESCRIPTION
The
\fImsgget\fR()
function operates on XSI message queues (see the Base Definitions volume of POSIX.1\(hy2017,
.IR "Section 3.226" ", " "Message Queue").
It is unspecified whether this function interoperates with the
realtime interprocess communication facilities defined in
.IR "Section 2.8" ", " "Realtime".
.P
The
\fImsgget\fR()
function shall return the message queue identifier associated with the
argument
.IR key .
.P
A message queue identifier, associated message queue, and data
structure (see
.IR <sys/msg.h> ),
shall be created for the argument
.IR key
if one of the following is true:
.IP " *" 4
The argument
.IR key
is equal to IPC_PRIVATE.
.IP " *" 4
The argument
.IR key
does not already have a message queue identifier associated with it,
and (\fImsgflg\fP & IPC_CREAT) is non-zero.
.P
Upon creation, the data structure associated with the new message queue
identifier shall be initialized as follows:
.IP " *" 4
.BR msg_perm.cuid ,
.BR msg_perm.uid ,
.BR msg_perm.cgid ,
and
.BR msg_perm.gid
shall be set to the effective user ID and effective group ID,
respectively, of the calling process.
.IP " *" 4
The low-order 9 bits of
.BR msg_perm.mode
shall be set to the low-order 9 bits of
.IR msgflg .
.IP " *" 4
.BR msg_qnum ,
.BR msg_lspid ,
.BR msg_lrpid ,
.BR msg_stime ,
and
.BR msg_rtime
shall be set to 0.
.IP " *" 4
.BR msg_ctime
shall be set to the current time, as described in
.IR "Section 2.7.1" ", " "IPC General Description".
.IP " *" 4
.BR msg_qbytes
shall be set to the system limit.
.SH "RETURN VALUE"
Upon successful completion,
\fImsgget\fR()
shall return a non-negative integer, namely a message queue identifier.
Otherwise, it shall return \-1 and set
.IR errno
to indicate the error.
.SH ERRORS
The
\fImsgget\fR()
function shall fail if:
.TP
.BR EACCES
A message queue identifier exists for the argument
.IR key ,
but operation permission as specified by the low-order 9 bits of
.IR msgflg
would not be granted; see
.IR "Section 2.7" ", " "XSI Interprocess Communication".
.TP
.BR EEXIST
A message queue identifier exists for the argument
.IR key
but ((\fImsgflg\fP & IPC_CREAT) && (\fImsgflg\fP & IPC_EXCL)) is
non-zero.
.TP
.BR ENOENT
A message queue identifier does not exist for the argument
.IR key
and (\fImsgflg\fP & IPC_CREAT) is 0.
.TP
.BR ENOSPC
A message queue identifier is to be created but the system-imposed
limit on the maximum number of allowed message queue identifiers
system-wide would be exceeded.
.LP
.IR "The following sections are informative."
.SH EXAMPLES
None.
.SH "APPLICATION USAGE"
The POSIX Realtime Extension defines alternative interfaces for interprocess communication
(IPC). Application developers who need to use IPC should design their
applications so that modules using the IPC routines described in
.IR "Section 2.7" ", " "XSI Interprocess Communication"
can be easily modified to use the alternative interfaces.
.SH RATIONALE
None.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "Section 2.7" ", " "XSI Interprocess Communication",
.IR "Section 2.8" ", " "Realtime",
.IR "\fIftok\fR\^(\|)",
.IR "\fImq_close\fR\^(\|)",
.IR "\fImq_getattr\fR\^(\|)",
.IR "\fImq_notify\fR\^(\|)",
.IR "\fImq_open\fR\^(\|)",
.IR "\fImq_receive\fR\^(\|)",
.IR "\fImq_send\fR\^(\|)",
.IR "\fImq_setattr\fR\^(\|)",
.IR "\fImq_unlink\fR\^(\|)",
.IR "\fImsgctl\fR\^(\|)",
.IR "\fImsgrcv\fR\^(\|)",
.IR "\fImsgsnd\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "Section 3.226" ", " "Message Queue",
.IR "\fB<sys_msg.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
